Projecting Climate Change Effects on Aspen Distribution and Productivity in the Central and Northern Rockies

In many vistas of the intermountain west, aspen feature prominently as the primary deciduous tree species, often in isolated stands on otherwise treeless landscapes or in stands surrounded by evergreens. Aspen’s iconic white bark is actually alive and capable of photosynthesis- an unusual adaptation that helps them to grow under a wide range of conditions.

It is due to adaptations like living bark that aspen enjoy the largest range of any tree in North America- one that extends from Alaska to Mexico and Vancouver to Maine, including almost every elevation in between. Aspen reproduce both sexually, like most trees, and clonally, forming extensive underground networks of roots. These networks can help them share water and nutrients widely from a single source and rebound quickly after a disturbance like fire or avalanche. Many aspen colonies have survived several major climatic fluctuations during their thousands of years on Earth. In fact, if you were looking for a species resilient under a wide range of environmental conditions, quaking aspen might seem like a good bet.

Nonetheless, following severe drought events, many aspen stands in the Rocky Mountains underwent a dramatic die-off during the last decade. Some low elevation populations suffered losses of up to 90 percent. To make matters worse, climate envelope models that project the future distribution of suitable habitat for a species suggest that aspen in the western U.S. may lose up to 40% of their habitat by as early as 2060.

This is bad news for natural resource managers because aspen groves serve as important ecological hotspots. Since they are often the only deciduous tree on a landscape, aspen increase species richness by providing critical habitat for animals like elk or species of songbirds that otherwise might not exist in the area.

When Tim Link, a hydrologist at the University of Idaho, learned about the ecological importance of aspen and the efforts of managers to actively sustain aspen for habitat, he saw the need to better understand how aspen respond to changing environmental factors. In some places managers had been fencing in aspen stands- an intensive management practice aimed at protecting aspen from grazing while allowing them to recover. But, as Link put it, “It probably doesn’t make sense to bother with stands that are simply not viable due to climate change.”

As a university hydrologist, Link was especially concerned with the effect that a climate-change induced shift from snow to rain would have on aspen growing in the northern Great Basin. At Reynolds Creek Experimental Watershed in southwestern Idaho, where Link does field work, such a shift is already well-documented and likely to continue. Less snow means fewer and smaller distributed snow drifts and, as a result, less reliable water availability for aspen in the spring and summer. How aspen will respond to this shift is unknown and has important implications for managers.

Meanwhile Doug Shinneman, a fire ecologist with the U.S. Geological Survey, is interested in another aspect of aspen’s sensitivity to climate change. With the switch from snow to rain, summers are expected to become drier and wildfires more common. “It’s uncertain what that will mean for aspen,” said Shinneman. “In parts of its range periodic fire may give aspen a competitive boost by knocking back conifers which would otherwise dominate the landscape. In other areas, such as low elevation or drought-prone regions, more fire might hasten the decline of aspen.”

In some regions, such as the Great Basin and the northern Rockies, there’s been very little published aspen-specific science. In the Great Basin aspen are often the only tree species around, so fire may not prove to be an advantage compared to where there are conifer competitors. “And actually that was one of the surprises,” said Shinneman, “it turns out that aspen’s relationships with fire are more complicated than commonly thought. In some places aspen can persist where there is adequate soil water and where it isn’t threatened by conifers, developing into multi-aged stands not dependent on fire.”

Teasing out more of the details of aspen’s complicated relationship to fire will help inform decisions such as whether or not to burn in order to maintain aspen groves- an important question for managers in places like northeastern Oregon where aspen groves provide critical habitat for elk.

After submitting independent proposals to the Northwest Climate Science Center (NW CSC) to study aspen, Link and Shinneman were invited to work together as co-PIs of a single project, given the similarity of their research questions. Despite its unusual inception, their collaboration has been a marked success, leading not only to a productive partnership among Link, Shinneman and NW CSC graduate fellow, Ben Soderquist, but also to additional research being carried out by other members of their teams, which include both university and government scientists. “Really the only downside has been that Doug is far away and I don’t get to sit down with him in person more.” said Link.

Another advantage of the NW CSC project has been to put the researchers in closer contact with a community of managers eager for the results of their study. Before submitting their proposal, Shinneman and Link had what Link called a “nascent” network of stakeholders- connections with managers with whom they had not previously worked directly. Their NW CSC grant acted as a catalyst to build those connections into active partnerships related, not just to aspen, but to juniper, to forest thinning, and to forest health more generally.

“Talking to people on the NRCS [U.S. Department of Agriculture’s Natural Resources Conservation Service] team, it turns out they have a lot of needs in the areas where we have expertise” said Link. “A really rewarding aspect of this project has been to hear from mangers about how much they value the work that we’re doing. And to have them help us spin our work into things that will be helpful to them. At our last meeting I had two people running out the door after me.”

Luckily the researchers are finding that it’s not all bad news for aspen. As Soderquist reported at the annual meeting of the American Geophysical Union in December, aspen show surprising resilience. “Last year was one of the lowest snow years on record in the Owyhee Mountains,” said Soderquist, “and while the trees did experience stress, it was not as much as we expected. They might be more resilient than we originally thought.” Better understanding the factors that support aspen resilience will help managers invest their efforts strategically so that our Northwest landscapes include aspen groves and the wildlife that they support for many decades to come.

Aspen is an environmentally, economically, and socially important species in the western U.S. It is typically the most abundant deciduous tree species in mountainous landscapes of the western U.S., providing food and habitat for a variety of wildlife, including black bear, deer, elk, moose, and numerous bird species. Aspen woodlands also provide high quality forage for livestock and draw tourists to the region to view the golden vistas that form in the fall. However, aspen is currently declining across large portions of the West, and it’s estimated that approximately 40% of western aspen will be without suitable climate conditions within 50 years. In the northern and central Rocky Mountains, it’s thought that reduced soil moisture and changing fire patterns in particular could affect the trees. Yet understanding the future of aspen in the region is currently challenged by a lack of information on how changes in fire and soil moisture interact to impact aspen growth, survival, and distribution. For example, in some places aspen may thrive following fire if soil moisture is adequate, while they may decline in areas where soil moisture is too low.

The goal of this work is to project the likely effects of altered moisture and fire regimes on aspen under climate change. Based on the results, researchers will develop maps of future aspen distribution in the northern and central Rocky Mountains and identify areas that could be suitable for aspen restoration. This information will help land managers prioritize areas for aspen protection and restoration and proactively plan for the effects of climate change on this important species.

Adaptive strategies for maintaining aspen-dominated woodlands: This project identifies adaptive strategies for addressing vulnerability and adaptation of aspen stands to shifting patterns of water availability and fire disturbances.

Provide maps of projected changes in aspen distribution: Maps identifying changes in aspen distribution under changing climate conditions will identify areas that are best suited for aspen restoration efforts and will offer insight to how treatments such as controlled burning will affect aspen productivity and ecological integrity as a whole.